Android强行进阶，自定义控件—LayoutManager，android开发视频

}
}

if (dx > 0) {
if (mHorizontalOffset >= getMaxOffset()) {
// 根据最大偏移量来计算滑动到最右侧边缘
mHorizontalOffset = (long) getMaxOffset();
dx = 0;
}
}

// 分离全部的view，加入到临时缓存
detachAndScrapAttachedViews(recycler);

float startX = 0;
float fraction = 0f;
boolean isChildLayoutLeft = true;

View tempView = null;
int tempPosition = -1;

if (onceCompleteScrollLength == -1) {
// 因为mFirstVisiPos在下面可能被改变，所以用tempPosition暂存一下
tempPosition = mFirstVisiPos;
tempView = recycler.getViewForPosition(tempPosition);
measureChildWithMargins(tempView, 0, 0);
childWidth = getDecoratedMeasurementHorizontal(tempView);
}

// 修正第一个可见view mFirstVisiPos 已经滑动了多少个完整的onceCompleteScrollLength就代表滑动了多少个item
firstChildCompleteScrollLength = getWidth() / 2 + childWidth / 2;
if (mHorizontalOffset >= firstChildCompleteScrollLength) {
startX = normalViewGap;
onceCompleteScrollLength = childWidth + normalViewGap;
mFirstVisiPos = (int) Math.floor(Math.abs(mHorizontalOffset - firstChildCompleteScrollLength) / onceCompleteScrollLength) + 1;
fraction = (Math.abs(mHorizontalOffset - firstChildCompleteScrollLength) % onceCompleteScrollLength) / (onceCompleteScrollLength * 1.0f);
} else {
mFirstVisiPos = 0;
startX = getMinOffset();
onceCompleteScrollLength = firstChildCompleteScrollLength;
fraction = (Math.abs(mHorizontalOffset) % onceCompleteScrollLength) / (onceCompleteScrollLength * 1.0f);
}

// 临时将mLastVisiPos赋值为getItemCount() - 1，放心，下面遍历时会判断view是否已溢出屏幕，并及时修正该值并结束布局
mLastVisiPos = getItemCount() - 1;

float normalViewOffset = onceCompleteScrollLength * fraction;
boolean isNormalViewOffsetSetted = false;

//----------------3、开始布局-----------------
for (int i = mFirstVisiPos; i <= mLastVisiPos; i++) {
View item;
if (i == tempPosition && tempView != null) {
// 如果初始化数据时已经取了一个临时view
item = tempView;
} else {
item = recycler.getViewForPosition(i);
}

addView(item);
measureChildWithMargins(item, 0, 0);

if (!isNormalViewOffsetSetted) {
startX -= normalViewOffset;
isNormalViewOffsetSetted = true;
}

int l, t, r, b;
l = (int) startX;
t = getPaddingTop();
r = l + getDecoratedMeasurementHorizontal(item);
b = t + getDecoratedMeasurementVertical(item);

layoutDecoratedWithMargins(item, l, t, r, b);

startX += (childWidth + normalViewGap);

if (startX > getWidth() - getPaddingRight()) {
mLastVisiPos = i;
break;
}
}
return dx;
}

涉及的方法：

/**

最大偏移量
@return
*/
private float getMaxOffset() {
if (childWidth == 0 || getItemCount() == 0) return 0;
return (childWidth + normalViewGap) * (getItemCount() - 1);
}

/**

获取某个childView在水平方向所占的空间，将margin考虑进去
@param view
@return
*/
public int getDecoratedMeasurementHorizontal(View view) {
final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams)
view.getLayoutParams();
return getDecoratedMeasuredWidth(view) + params.leftMargin

params.rightMargin;
}

/**

获取某个childView在竖直方向所占的空间,将margin考虑进去
@param view
@return
*/
public int getDecoratedMeasurementVertical(View view) {
final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams)
view.getLayoutParams();
return getDecoratedMeasuredHeight(view) + params.topMargin

params.bottomMargin;
}

回收复用

这里使用Android仿豆瓣书影音频道推荐表单堆叠列表RecyclerView-LayoutManager中使用的回收技巧：

/**

@param recycler
@param state
@param delta
*/
private int fill(RecyclerView.Recycler recycler, RecyclerView.State state, int delta) {
int resultDelta = delta;
//。。。省略

recycleChildren(recycler);
log(“childCount= [” + getChildCount() + “]” + “,[recycler.getScrapList().size():” + recycler.getScrapList().size());
return resultDelta;
}

/**

回收需回收的Item。
*/
private void recycleChildren(RecyclerView.Recycler recycler) {
List<RecyclerView.ViewHolder> scrapList = recycler.getScrapList();
for (int i = 0; i < scrapList.size(); i++) {
RecyclerView.Vi
ewHolder holder = scrapList.get(i);
removeAndRecycleView(holder.itemView, recycler);
}
}

回收复用这里就不验证了，感兴趣的小伙伴可自行验证。

动画效果

// 缩放子view
final float minScale = 0.6f;
float currentScale = 0f;
final int childCenterX = (r + l) / 2;
final int parentCenterX = getWidth() / 2;
isChildLayoutLeft = childCenterX <= parentCenterX;
if (isChildLayoutLeft) {
final float fractionScale = (parentCenterX - childCenterX) / (parentCenterX * 1.0f);
currentScale = 1.0f - (1.0f - minScale) * fractionScale;
} else {
final float fractionScale = (childCenterX - parentCenterX) / (parentCenterX * 1.0f);
currentScale = 1.0f - (1.0f - minScale) * fractionScale;
}
item.setScaleX(currentScale);
item.setScaleY(currentScale);
item.setAlpha(currentScale);

layoutDecoratedWithMargins(item, l, t, r, b);
// 省略 …
}
return dx;
}

childView 越向屏幕中间移动缩放比越大，越向两边移动缩放比越小。

自动选中

1、滚动停止后自动选中

监听 onScrollStateChanged，在滚动停止时计算出应当停留的 position，再计算出停留时的 mHorizontalOffset 值，播放属性动画将当前 mHorizontalOffset 不断更新至最终值即可。相关代码如下：

@Override
public void onScrollStateChanged(int state) {
super.onScrollStateChanged(state);
switch (state) {
case RecyclerView.SCROLL_STATE_DRAGGING:
//当手指按下时，停止当前正在播放的动画
cancelAnimator();
break;
case RecyclerView.SCROLL_STATE_IDLE:
//当列表滚动停止后，判断一下自动选中是否打开
if (isAutoSelect) {
//找到离目标落点最近的item索引
smoothScrollToPosition(findShouldSelectPosition());
}
break;
default:
break;
}
}

/**

平滑滚动到某个位置
@param position 目标Item索引
*/
public void smoothScrollToPosition(int position) {
if (position > -1 && position < getItemCount()) {
startValueAnimator(position);
}
}

private int findShouldSelectPosition() {
if (onceCompleteScrollLength == -1 || mFirstVisiPos == -1) {
return -1;
}
int position = (int) (Math.abs(mHorizontalOffset) / (childWidth + normalViewGap));
int remainder = (int) (Math.abs(mHorizontalOffset) % (childWidth + normalViewGap));
// 超过一半，应当选中下一项
if (remainder >= (childWidth + normalViewGap) / 2.0f) {
if (position + 1 <= getItemCount() - 1) {
return position + 1;
}
}
return position;
}

private void startValueAnimator(int position) {
cancelAnimator();

final float distance = getScrollToPositionOffset(position);

long minDuration = 100;
long maxDuration = 300;
long duration;

float distanceFraction = (Math.abs(distance) / (childWidth + normalViewGap));

if (distance <= (childWidth + normalViewGap)) {
duration = (long) (minDuration + (maxDuration - minDuration) * distanceFraction);
} else {
duration = (long) (maxDuration * distanceFraction);
}
selectAnimator = ValueAnimator.ofFloat(0.0f, distance);
selectAnimator.setDuration(duration);
selectAnimator.setInterpolator(new LinearInterpolator());
final float startedOffset = mHorizontalOffset;
selectAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
@Override
public void onAnimationUpdate(ValueAnimator animation) {
float value = (float) animation.getAnimatedValue();
mHorizontalOffset = (long) (startedOffset + value);
requestLayout();
}
});
selectAnimator.start();
}

2、点击非焦点view自动将其选中为焦点view

我们可以直接拿到 view 的 position，直接调用 smoothScrollToPosition 方法，就可以实现自动选中为焦点。

中间view覆盖在两边view之上

效果是这样的：

从效果中可以看出，索引为2的view覆盖在1，3的上面，同时1又覆盖在0的上面，以此内推。

RecyclerView 继承于 ViewGroup ，那么在添加子view addView(View child, int index) 中 index 的索引值越大，越显示在上层。那么可以得出，为2的绿色卡片被添加是 index 最大，分析可以得出以下结论：

index 的大小：

0 < 1 < 2 > 3 > 4

中间最大，两边逐渐减小的原则。

获取到中间 view 的索引值，如果小于等于该索引值则调用 addView(item) ，反之调用 addView(item, 0) ；相关代码如下：

private int fillHorizontalLeft(RecyclerView.Recycler recycler, RecyclerView.State state, int dx) {
//省略 …
//----------------3、开始布局-----------------
for (int i = mFirstVisiPos; i <= mLastVisiPos; i++) {
//省略 …
int focusPosition = (int) (Math.abs(mHorizontalOffset) / (childWidth + normalViewGap));
if (i <= focusPosition) {
addView(item);
} else {
recycler, RecyclerView.State state, int dx) {
//省略 …
//----------------3、开始布局-----------------
for (int i = mFirstVisiPos; i <= mLastVisiPos; i++) {
//省略 …
int focusPosition = (int) (Math.abs(mHorizontalOffset) / (childWidth + normalViewGap));
if (i <= focusPosition) {
addView(item);
} else {